Florence, Arizona - An Arizona man convicted of the murders of four members of a Phoenix family was executed by lethal injection on Friday, the fourth execution in the US this week.

Richard Djerf (55) was pronounced dead at 10:40 AM local time (1:40 PM ET) at a state prison in the town of Florence and had no last words, an Arizona Department of Corrections spokesman said.

Djerf was sentenced to death for the 1993 murders of four members of the family of a co-worker he had accused of theft.

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In a letter last month apologizing for the crime, Djerf said he was ready to die and would not seek clemency.

"If I can't find reason to spare my life, what reason would anyone else have?" he wrote.

Two convicted murderers were put to death on Tuesday – in Florida and Missouri – and another inmate was executed on Wednesday in Mississippi.

There have been 39 executions in the US this year, the most since 2013, when 39 inmates were also put to death.

Florida has carried out the most executions with 14, followed by Texas with five and South Carolina and Alabama with four.

Thirty-three of this year's executions have been carried out by lethal injection, two by firing squad, and four by nitrogen hypoxia, which involves pumping nitrogen gas into a face mask, causing the prisoner to suffocate.

United Nations experts have denounced the use of nitrogen gas as a method of capital punishment as cruel and inhumane.

The death penalty has been abolished in 23 states, while three others – California, Oregon, and Pennsylvania – have moratoriums in place.